Hydrogen retention and release dynamics of amorphous carbon films exposed to a hydrogen plasma

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Abstract
H2 and DH release was investigated in a pulsed deuterium discharge over a carbon thin-film deposited wall. A large quantity of hydrogen was desorbed by 300 eV D+2 irradiation from hydrogenated amorphous carbon (a-C:H) deposited by a hydrogen-admixed methane discharge. On the other hand, a strong pumping effect was observed in the case of films deposited by a helium-admixed discharge. The wall pumping effect was attributed to the presence of interstitial vacancies rather than dangling bonds in the films. A linear dependence of the hydrogen release rate on the ion bombarding current suggests that molecular formation is by direct hydrogen abstraction instead of recombination of two free hydrogen atoms. A dynamic model of hydrogen release and retention for a-C:H layers during hydrogen implantation is proposed. Corresponding calculations yield satisfactory agreement with observations when using appropriate rate constants for trapping and detrapping.